The MX and A records still point to my mail server, as they have for the past 10 years. If I'm relaying through Gmail, I don't need to them, do I ?

I also called my ISP and they said:
- they are blocking port 25 both inbound and outbound
- If I set up SSL, inbound is port 995 and outbound is 465
- If no SSL, inbound is port 110 and outbound is 587

that is your problem , outside mail servers will always try to comunicate with your MX on port 25 , if you ISP is blocking it then you wont get any mail
ask them if they provide a smart host that you can use for relaying mail